Magnetorheological fluid dampers have found a number of practical applications in automotive suspensions, clutches, engine mounts, vibration control units, earthquake proofing equipment, and robotic systems. The magnetorheological fluid in the damper changes key rheological properties, such as yield stress or viscosity, in response to a magnetic flux to adjust the damping characteristics of the damper.
FIG. 1 shows a cutaway perspective view for a magnetorheological (MR) piston including a piston core. Magnetorheological (MR) dampers have a cylinder (not shown) containing an MR fluid and an MR piston 10 slidably engaging the inner diameter of the cylinder. In this example, the MR fluid passes through a flow gap 12 between the inner surface of solid piston ring 14 and the outer surface formed by piston core 16 and coil winding 18. The magnetic field in the flow gap 12 is changed by varying the electric current in the coil winding 18, which changes the yield stress of the MR fluid in the flow gap 12. This changes the damping characteristics of the MR damper. A rod 20 is attached to the MR piston 10 and extends outside the cylinder. The cylinder and the rod 20 are attached to separate structures to dampen relative motion of the two structures along the direction of MR piston travel.
FIG. 2, in which like elements share like reference numbers with FIG. 1, shows a magnetic flux density distribution plot for a magnetorheological (MR) piston including a piston core. The magnetic flux density in the piston core 16 includes a high flux density region 22 and a low flux density region 24. The high flux density region 22 is typically located between the longitudinal axis of the piston core 16 and the coil winding 18. When the material in the high flux density region 22 is magnetically saturated, the flux density in the flow gap 12 is limited, regardless of the electric current through the coil winding 18. The high flux density region 22 restricts the magnetic flux through the central portion of the core, acting as a flux bottleneck, and thus limits the dynamic range and performance of the MR damper.
Several approaches have been implemented or suggested to work around the problem of limitation of the flux density in the flow gap due to magnetic saturation, using changes to the piston core materials, the piston core geometry, or the MR fluid.
One approach has been to build the whole piston core from a high-performance magnetic alloy which saturates at a flux density higher than that encountered in the MR damper. The cost of suitable high-performance magnetic alloys, such as Cobalt steel and Vanadium/Cobalt steel (Permendur), greatly exceeds the cost of low-carbon steel used presently. The increased cost makes this approach uneconomical for mass-produced items, such as automotive dampers, which are produced in large numbers and for which even a small fractional cost determines profit or loss.
Another approach has been to change the piston core geometry to increase the flux density in the flow gap, such as by reducing the width of the flow gap. This increases the flux density in the flow gap for a given number of ampere-turns in the coil winding, but precludes desirable damper configurations. The flow resistance of the flow gap depends on its width, so reducing the width of the flow gap increases flow resistance. Flow resistance at low or no coil winding current is higher than desirable, precluding this approach.
Yet another approach has been to increase the iron content of the MR fluid to increase its yield stress for a given flux density in the flow gap. This causes a number of materials problems, such as particle separation, particle sedimentation, increased abrasion, and increased viscosity. The increased iron content causes operational difficulties, such as greater magnetic field loss and reduction in damper dynamic range. The higher viscosity also requires larger flow gap widths in order to maintain acceptable low damping forces when the coil current is low or zero. The required increased gap width in turn reduces the flux density in the flow gap, thus negating the benefits of increased iron content in the fluid. Increased iron content also increases MR fluid cost. The many problems resulting from increased iron content in the MR fluid make this approach undesirable.
Accordingly, it would be desirable to have a high-performance piston core for a magnetorheological damper that overcomes the disadvantages described.